The invention relates to a switched reluctance (“SR”) motor. More particularly, the invention relates to a method and system of regulating the speed of an SR motor.
SR motors have multiple poles on both the stator and the rotor. There are windings or coils wound on the stator poles, and each pair of windings on diametrically opposite stator poles is connected in series to form an electrically independent phase of the SR motor. There are no windings or magnets on the rotor. However, the rotor is made of a magnetically permeable material such as, for example, a ferrous alloy. Each rotor also has a rotor position relative to the stator. To run a SR motor efficiently, it is necessary to determine the rotor position with respect to the stator. The rotor position also establishes which phase of the stator or a firing angle or a turn-on angle is to be energized or commutated first by applying different amounts of current or voltage in the windings. If the rotor position is incorrect, commutation of one of the stator phases may result in inefficient or improper operation of the motor.
A typical SR motor controller uses a peak detection method to control the firing angle or the turn-on angle in a sensorless operation. Particularly, the controller uses the peak detection method to locate peaks from the different amounts of current commanded by the motor. However, the peak detection method requires that a maximum voltage (that varies with speed) not be exceeded. When the maximum voltage is exceeded, peaks will not be detected. As a result, the rotor position will be lost. If the rotor position is lost, the controller loses track of where voltage or current should be applied, and subsequently shuts down the SR motor. Inability to exceed the maximum voltage limit also prevents the SR motor from generating full torque at low speed or near zero speed. Furthermore, the sluggishness of the voltage control also prevents the fast dynamic response needed at the lower speeds.